In such systems, the information is often in the form of binary data represented by pulses modulating an optical carrier wave. A binary value is thus determined as a function of the amplitude (or power) level of the modulated optical wave.
During transmission, the signal can be subjected to degradation that makes it more difficult for receivers to detect the high levels and the low levels of the received signal.
In the amplitude domain, the quality of an optical signal is usually defined by two parameters: the signal-to-noise ratio and the extinction ratio.
The signal-to-noise ratio is defined as the ratio of the optical power of the signal to the noise power in a wavelength band including the wavelength of the carrier of the signal.
The extinction ratio is defined as the ratio of the power corresponding to the high level of the signal divided by the power corresponding to the low level of the signal. This ratio must be high enough in spite of variations in the input signal.
Optical signal wavelength converters are used in telecommunications to convert the transmitted optical signal from one wavelength to another wavelength while retaining signal performance.
Such changes in wavelength are used in particular when routing the signals to solve problems of conflict.
Thus, a converter must be capable of using a poor-quality modulated input signal to deliver an output signal whose high levels are stabilized to a constant optical power, and whose low levels have almost zero power, while also having high signal-to-noise ratio.
One possible solution for increasing the extinction ratio when making wavelength converters consists in using an interferometer structure of the Mach-Zehnder type or of an equivalent type.
Such a structure is shown in FIG. 1 and is made up of two branches conveying two coherent waves coupled together to form the output signal. One of the branches comprises a medium whose refractive index varies as a function of the optical power that it conveys, and an input signal is fed into this branch. Variations in the power of the input signal then modulate the refractive index, and the two waves interfere either destructively or constructively as a function of the power level of the input signal.
Such a structure does indeed make it possible to improve the extinction ratio, but it suffers from the drawback that the conditions for destructive and constructive interference are very restrictive on the input signal, in particular with respect to its wavelength, its power level in the high state, and its polarization.
Operation of wavelength converters based on Mach-Zehnder type interferometer structures having semiconductor optical amplifiers is thus very sensitive to fluctuations in those parameters.